Mexico Doug, Bernd and List, Bernd has been leading a few of us, off list, in a discussion of Acapulcoites v Lodranites. Doug's fine list of references, including David Weir's wonderfully up to date pages, shows the likelihood that a more comprehensive classification system will evolve. Grain size is an artifact of metamorphism and correlates well but tells the story only incompletely and imperfectly.
A while back I put up a page with a few pictures of winonaites and acapulcoites in thin section. Now these classifications are clearly distinguished with olivine and oxygen isotope composition. Still, there are pictures of one former and two current acapulcoites there where grain sizes can be compared. If anyone wants to check out another guy's picture of NWA 2871 for grain sizes I just posted a large version of one of mine. Just click on the last picture on that page for a new page or use the second link below. Do remember, though, that most grain portions we see in thin section do not represent the full size of the grain since the plane of the section will cut through the thickest portion of only a small percentage of the grains it encounters. http://johnkashuba.com/Pages/Meteorite%20Pages/Topics/Acapulcoites_and_Winon aites.html http://johnkashuba.com/Pages/Meteorite%20Pages/Pictures/NWA2871Acapulcoite.h tm Best regards, - John John Kashuba Ontario, California -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Monday, November 05, 2007 1:41 PM To: meteorite-list@meteoritecentral.com Subject: [meteorite-list] NWA 2871: Rocks From Space Picture of the Day - November 5, 2007 Doug kindly wrote: "Just a click away and an interested person without the materials and microscope can vicariously participate with Bernd's clear and colorful image of his nice thin section of Lodranite/Acapulcoite material." Thank you, Doug. Much appreciated! Yes, that's one of my best pics that I've taken so far in cross-polarized light and still without a gadget to firmly attach the camera to the microscope, camera only hand-held! Doug: "Bernd, could you kindly mention which of the grains were measured for those of us who want to confirm our little fun with a tape measure on our computer screens?" I could but I won't ;-) Let me explain my rudeness: You have the field of view in the upper right corner, so it's easy to figure out the rest - an easy exercise! Doug quoting from C. Floss: "The simple bimodal classification of these meteorites based primarily on petrographic criteria, which has been used to date, appears to be inadequate to describe this diverse group of samples, ..." Hence I wrote and also quoted on Friday, Oct 26: ---------------------------------------------------------------------------- ------------- But: It is not only grain size! ... and that's why this excerpt from an article in MAPS may be of interest in this context: TERRIBILINI D. et al. (2000) Evidence for common breakup events of the acapulcoites-lodranites and chondrites (MAPS 35-5, 2000, pp. 1043-1050). "GRA 95209 is considered a lodranite based on petrography (McCoy and Carlson, 1998), but Mittlefehldt and Lindstrom (1998) find that the bulk-rock composition is more similar to acapulcoites and quite different from that of lodranites. Thus, a clear-cut classification is difficult." ---------------------------------------------------------------------------- ------------- Best wishes, Bernd ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list